We generally utilise OneTrust Cookie Consent the implementation into any solution is very simple, very customizabe and generally just works well. 

On our CMS11 and 12 solutions we have a concept of Script Blocks properties with Inheritance where any Third Party Scripts can be added easily by Content Cloud Editors. But you can also add OneTrust via Google Tag Manager although we have found some conflicts previously doing it this way.   

There are several cookie consent solutions that are compatible with Optimizely X. In general, I use Cookiebot, this is a comprehensive cookie consent and management platform that is fully integrated with Optimizely X. It allows you to customize your cookie consent banner, manage your cookie inventory, and comply with GDPR and other privacy regulations.

One we've used recently for client sites is Cookie Information. As with the others mentioned, it has a good SDK, is very configurable (from pre-defined templates, to complete customizable HTML/CSS/JS). All text in the banner / cookie list can be customized. All cookies on the site are automatically scanned, classified and explained - but you can also override any definitions with your own custom ones.

There's also a ready-made Optimizely integration available: https://support.cookieinformation.com/en/articles/4621447-optimizely-previously-episerver-integration

Other than the ones mentioned above we have also integrated with Osano (https://www.osano.com/)

Most of the integrations include embedding a script in the HTML or via GTM and are not restricted (usually) by the version of Optimizely.